PDE4 is one of the major phosphodiesterases expressed in leukocytes. Inhibitors of PDE4 cause accumulation of intracellular cyclic adenosine monophosphate (cAMP), which in turn activates protein kinase A and other downstream effectors, resulting in inhibition of pro-inflammatory cytokine transcription and other cellular responses such as neutrophil degranulation, chemotaxis, and adhesion to endothelial cells.